2.1. HYBRIDIZATION
Hybridization depends on the pairing of complementary bases in nucleic acids and is a specific tool useful for the general recognition of informational polymers. Diverse research problems using hybridization of synthetic oligonucleotide probes of known sequence include, amongst others, the different techniques of identification of specific clones from cDNA and genomic libraries; detecting single base pair polymorphisms in DNA; generation of mutations by oligonucleotide mutagenesis; and the amplification of nucleic acids in vitro from a single sperm, an extinct organism, or a single virus infecting a single cell.
It is possible to discriminate perfect hybrids from those hybrids containing a single internal mismatch using oligonucleotides 11 to 20 nucleotides in length [Wallace et al., Nucl. Acids Res. 6: 3543 (1979)]. Mismatched hybrids are distinguished on the basis of the difference ill the amount of hybrid formed in the hybridization step and/or the amount remaining after the washing steps [Ikuta et al., Nucl. Acids Res. 15: 797 (1987); Thein and Wallace, in Human Genetic Diseases: A Practical Approach, ed. by J. Davies, IRL Press Ltd., Oxford, pp. 33-50 (1986)].
The reproducible hybridization of different and diverse short oligonucleotides less than 11 nucleotides long has not been well characterized previously. Detailed hybridization data that allows a constant set of conditions for all predictable oligonucleotides is not available [Besmer et al., J. Mol. Biol. 72: 503 (1972); Smith, in Methods of DNA and RNA Sequencing, ed. S. Weissman, Praeger Publishers, New York, N.Y., pp. 23-68 (1983); Estivill et al., Nucl. Acids Res. 15: 1415 (1987).
Information is also not available on the effects of a single noncomplementary base pair located at the 5' or 3' end of a hybridizing oligonucleotide that produces a mismatched hybrid when associated with a target nucleic acid. Hybridization conditions that discriminate between (1) a perfectly complementary hybridizing pair of nucleic acid sequences where one partner of the pair is a short oligonucleotide, and (2) a pair wherein a mismatch of one nucleotide occurs on the 5' or 3' end of the oligonucleotide, provide a more stringent environment than is required for internal mismatches because hybrid stability is affected less by a mismatch at the end of a hybridizing pair of complementary nucleic acids than for an internal mismatch.
The length of nucleotides that can distinguish a unique sequence in a nucleic acid of defined size has been predicted [Smith in Methods of DNA and RNA Sequencing, ed. S. Weissman, Praeger Publishers, New York, N.Y., pp. 23-68 (1983)]. Thus random oligonucleotide sequences 16-17 long are expected to occur only once in random DNA of 3.times.10.sup.9 bp, the size of the human genome. However, with decreasing probe length, e.g. for oligonucleotides 5 to 10 nucleotides in length, there is an exponential increase in the frequency of occurrence within a random DNA of a given size and complexity. Thus, the purposes for which oligonucleotide probes are employed can impact on the length of the oligonucleotides that are used experimentally.